1. Field of the Invention
The present invention relates to a projection type display apparatus used for video display, large-screen display, and so on.
2. Related Background Art
In recent years the variety of use of display apparatus has increased in the fields of image processing etc. and thus there are needs for display devices capable of implementing optimum chromatic purity, chromatic balance, illuminance, and so on according to their use.
FIG. 10 shows an example of the conventional projection type display apparatus.
In the figure, white light emitted from a light source unit 1 having a reflector 2 travels through fly-eye lenses 3 and 4, a PS converter 5, a condenser lens 6, etc. and thereafter a dichroic mirror DM1 as a first optical system transmits red band light R but reflects green and blue band light. Then the red band light transmitted by the dichroic mirror DM1 is reflected by a total reflection mirror M1 to change its optical path by 90.degree. and it then travels through a field lens 7R and a trimming filter TR into a red liquid-crystal display unit 8R comprised of an image display element, in which the red band light is optically modulated according to an input signal. The light thus optically modulated is incident to a dichroic prism 9 for combining as a second optical system, and the dichroic prism 9 changes the optical path of the red light by 90.degree. into a projection lens 10.
On the other hand, among the green and blue band light reflected by the dichroic mirror DM1 so as to change its optical path by 90.degree., the green band light G is reflected by another dichroic mirror DM2 also acting as a first optical system to change its optical path by 90.degree. and then it travels through a field lens 7G and a trimming filter TG into a green liquid-crystal display unit 8G comprised of an image display element, in which the green band light is optically modulated according to an input signal. The light thus optically modulated is incident to the dichroic prism for combining 9 and then into the projection lens 10. Further, the blue band light B transmitted by the dichroic mirror DM2 travels via a condenser lens 11, a total reflection mirror M2, a relay lens 12, a total reflection mirror M3, and a field lens 7B into a blue liquid-crystal display unit 8B comprised of an image display element, in which the blue band light is optically modulated according to an input signal. The light thus optically modulated is incident to the combining dichroic prism, and the dichroic prism 9 changes the optical path of the blue light by 90.degree. into the projection lens 10.
Then trichromatic light combined by the combining dichroic prism is projected by the projection lens 10.
A light source of the light source unit 1 is normally selected from a halogen lamp, a metal halide lamp, UHP (available from Philips Inc.: registered trademark), and so on, the dichroic mirrors DM1 and DM2 have their spectral transmittances illustrated in FIG. 11A and FIG. 11B, respectively, and the trimming filters TR and TG have their spectral transmittances as illustrated in FIG. 11C and FIG. 11D, respectively.
In the conventional projection type display apparatus constructed as described above, where the contrast was sufficiently high in each of the liquid-crystal panels, the chromatic purity of the red band light was determined by the spectral property of the light-source light and the spectral transmittances of the dichroic mirror DM1 and the trimming filter TR, the chromatic purity of the green band light was determined by the spectral property of the light-source light and the spectral transmittances of the dichroic mirrors DM1, DM2 and the trimming filter TG, the chromatic purity of the blue band light was determined by the spectral property of the light-source light and the spectral transmittances of the dichroic mirrors DM1, DM2, and the optical elements were designed so as to gain the optimum chromatic balance and chromatic purity.
Incidentally, there are such various demands that bright display is required even with a penalty of size increase of the apparatus, that bright display is required even with a penalty of degradation of chromatic purity, that the chromatic purity is not required in a white/black mode, and so on, depending upon use purposes of the projection type display apparatus.
The convectional projection type display apparatus implemented the optimum chromatic balance and chromatic purity by cutting the light in the wavelength region of 570 nm to 600 nm in order to enhance the chromatic purity.
However, many lamps have a peak near 580 nm in their spectral distribution, and use of wavelengths near it contributes to increase of illuminance.
There is thus the known projection type display apparatus with a mechanism for putting a band cut filter as an optical element (DF) capable of varying the chromatic purity into or out of the optical path, which is described in Japanese Patent Application Laid-Open No. 7-72450.
This projection type display apparatus was a single device capable of implementing both brightness-priority display and color-reproduction-priority display, but it had a problem that in the color-reproduction-priority display the display became darker than necessary, because the band cut filter was likely to lower the transmittance even at the wavelengths necessary for the projection.